The following discussion will make explicit reference, to the generation of a load current for electrical testing of an electronic device, such as, for example, a charge pump. As it is known, charge pumps are used in a wide range of applications for supplying boosted voltages, starting from input voltages of a value lower than a desired value. For example, in memory devices, charge pumps supply the voltages required for the operations of modifying, programming, or erasing, which have high values, higher than the value of the supply voltage available in the same memory devices.
During electrical testing of a charge pump integrated in a body of semiconductor material, such as silicon, it is desired to obtain its voltage/current characteristic curve, as the temperature, clock frequency, and supply voltage vary. This electrical test is, for example, envisaged for the step of validation of the memory device in which the charge pump is used, a step that uses characterization of all the analog circuit blocks internal to the device, both low-voltage ones (for example, oscillators, band-gap reference circuits, etc.), and high-voltage ones (for example, the charge pumps).
For this purpose, a known load current is generally set, the value of which is appropriately varied during the testing operation, and the value of the output voltage generated by the charge pump is measured.
In this way, the voltage/current characteristic of the charge pump is obtained, as illustrated by way of example in FIG. 1, where the variation of the non-regulated output voltage Vout of the charge pump is shown as a function of the load current IL. The output voltage is between a maximum value Vout—max and a minimum value Vout—min. FIG. 1 also shows the regulated output value Vreg (ON/OFF type regulation) of the charge pump. In the case illustrated, referred to the use in a memory device of a Flash type, the load current IL has, for example, a value on the order of several milliamps.
The value of the load current should remain stable as the supply voltage, the temperature, and the parameters of the manufacturing process (the so-called “process spread”) vary, in particular owing to the so-called “process corners”, i.e., the limit or worst cases.
Known approaches for electrical testing of a charge pump in a memory device envisage the load current IL to be set, either inside the memory device or outside the memory device, via a testing machine or testing device.
As shown schematically in FIG. 2, in the case of internal generation of the load current in a memory device, designated as a whole by reference numeral 1, the charge pump, designated by 2 is coupled, inside the memory device 1 (i.e., in the same chip of semiconductor material), to a current generator 4, which supplies the load current IL, of a value that is known and is set beforehand, and which may vary in a desired way for the purposes of the voltage/current characterization.
The storage device 1 further includes a transfer stage 5 including a plurality of pass transistors to transfer to a testing device, designated by 6 (the so-called “tester”), the output voltage Vout generated by the charge pump 2.
As shown in FIG. 3, the current generator 4 has a current-mirror configuration, where, starting from a reference current Iref, the load current IL is generated at the output by a plurality of current-mirror stages S0 . . . Sn enabled by respective enable bits B0 . . . Bn. Each current-mirror stage S0 . . . Sn supplies on an internal node N1 a current equal to the reference current Iref multiplied by a factor 2i (i being an index that ranges from 0 to n).
The current generator 4 further comprises an output stage 6, which is also provided by a current mirror having an output transistor 8, which supplies on a drain terminal thereof the load current IL.
This known approach has, however, some drawbacks. In the first place, it requires a considerable area occupation in the integrated realization, on account of the large number of current-mirror stages required for boosting the input current (even by a factor of 100 or higher).
Moreover, this approach does not guarantee a sufficient precision for many applications in so far as the exact value of the load current IL depends upon the output voltage Vout of the charge pump 2 (see also FIG. 2). In fact, the output voltage Vout is the biasing voltage present on the drain terminal of the aforesaid output transistor 8.
In a further approach of a known type, shown in FIG. 4, the load current IL for electrical testing of the charge pump, designated once again by 2, is generated outside the memory device 1, by the testing device 6. However, also this approach is not free from drawbacks.
In particular, an imprecision arises in reading the output voltage Vout generated by the charge pump 2, which is altered by the drop on the pass transistors in the transfer stage 5 inside the memory device 1, which is used to enable the measurement path, which is traversed by the load current IL.